1. Field of the Invention
This invention relates to a novel, multi-purpose electrolytic cell.
2. Description of Related Prior Art
Filter press bipolar electrolyzers are known. These have a bipolar wall or backplate separating the cathodic compartment from the anodic compartment of adjacent cell units in a series arrangement of the unit cells. On one side of the bipolar wall or backplate is the cathode structure and on the other side the anode structure. When multiple bipolar cell frames are connected in series to form the electrolyzer, an anode endplate and a cathode endplate is utilized at each end of the series to apply appropriate pressure to hold the units of the series together.
In electrochemical processes in which the anolyte and the catholyte and the respective electrolysis products must be kept separated, a permeable diaphragm or a semi-permeable membrane or a permselective membrane is positioned between the anode of a bipolar element and the cathode of an adjacent bipolar element. Electrical continuity between the anode of one unit in the series of bipolar elements and the cathode of an adjacent cell unit in the series is provided across the bipolar wall or backplate. The bipolar wall or backplate is, accordingly, cathodically polarized and in contact with the catholyte on one side and anodically polarized and in contact with the anolyte on the other side of the backplate. Consequently, the two surfaces of the bipolar wall or backplate may exhibit quite different corrosion resistance properties as a result of the use of different electrolyte and electrolysis products in contact therewith.
In prior art bipolar electrolyzers, the backplate is considered to have three functions. First, the backplate separates the catholyte of one bipolar cell from the anolyte of the adjacent bipolar cell of the electrolyzer. Second, the backplate serves as a conductive member connecting the cathode of one unit of the bipolar electrolytic cell with the anode of an adjacent cell of the bipolar electrolyzer. Third, the backplate acts as a structural member since both anodes and cathodes extend substantially perpendicular to the backplate.
Bipolar cells in which titanium or other valve metals are used as anodes in processes in which hydrogen is evolved from the cathode surface are subject to the disadvantage that during electrolysis nascent hydrogen, which is formed at the metal cathode surface, permeates through the metal cathode and attacks the titanium or other valve metal, on the anode side of the bipolar electrode. Titanium hydride is formed which can be the cause of blistering, embrittlement, flaking, misalignment, and stress cracking of the anode. Hydrogen continues to permeate through the titanium hydride thus formed which results in a further formation of titanium hydride and further deterioration of the anode. Deterioration of titanium anodes significantly decreases the useful life of bipolar electrodes, contaminates the products produced by bipolar cells, and increases the cost of operating the cell. Although other materials can be used in place of iron or steel for the cathode portion of the electrode, most metals that are useful are also permeable to hydrogen to some extent.
Filter press electrolyzers having cell units assembled utilizing a molded thermoplastic polymer filter press frame are known. Such filter press electrolyzers are known utilizing an injection molded plastic frame enclosing a chamber for electrolyte therebetween, as shown in U.S. Pat. No. 5,421,977 and references cited therein.
In U.S. Pat. No. 5,082,543 to Gnann et al, a filter press electrolysis cell is disclosed for the production of peroxy and perhalogenate compounds including peroxydisulfates and peroxydisulfuric acid. Platinum coated valve metal substrates are disclosed as anodes, the platinum layer being applied to the substrates by hot isostatic pressing or diffusion welding of a platinum foil onto the valve metal substrate. Preferably, the platinum foil has a thickness of about 20 to about 100 microns. The cathode used in the electrolytic cell is a perforated, liquid and gas permeable cathode of stainless steel which is further identified as tool steel number 1.4539. Electrolysis cell separators are disclosed as cation exchange membranes such as Nafion.RTM. 423. These are clamped between the frames of the cell and the frames are sealed by gaskets of a vinylidene fluoride-hexafluoropropylene copolymer.
In the filter press electrolysis cell described in U.S. Pat. No. 5,082,543 to Gnann et al., hollow cathodes and anodes are disclosed wherein the cathode hollow bodies are liquid and gas permeable and the anode hollow bodies have, above and below a platinum layer, openings for the introduction and removal of the anolyte. The effective anode surface is formed by the platinum layer of a composite anode comprising a valve metal substrate and a platinum layer present thereon which is obtainable by the hot isostatic pressing of a platinum foil onto a valve metal substrate. The cells of this reference are disclosed as useful for the production of peroxy compounds, specifically, the anodic production of peroxydisulfate, peroxomono sulfates, and peroxydiphosphates. By providing circulation of cooling water in the anode, the electrolysis operation is disclosed as being able to proceed with current densities of up to 15 kA/m.sup.2 by reducing ohmic voltage losses caused by heating of the anode surface.
Gnann et al. in the '543 patent discloses an electrolysis cell having an anode hollow body and a cathode hollow body through which cooling water circulates in order to dissipate heat formed, particularly, in the anodic production of peroxydisulfates and salts thereof. Because such a cell design in which hollow electrodes are used is fraught with the danger of leakage of the cooling water into the cell electrolyte and, accordingly, requires effective, dependable sealing so as to avoid such leakage, with the possibility of precipitation of one or more electrolysis products within the cell, such a cell design has been intentionally avoided in favor of the use of external heat exchangers in the process of the invention.